When rain falls and snow melts, water ends up on the ground surface. This water is called stormwater. Stormwater can soak into the ground, evaporate, or runoff into nearby bodies of water. In cities, stormwater often runs of into nearby bodies of water because it can't sink into the ground due to impervious surfaces, such as roads, roofs, and sidewalks. As stormwater travels over these impervious surfaces it can pick up pollutants such as litter, animal waste, oil from spills, fertilizer from lawns, and leaves, which can then lead to the pollution of surface water. To reduce this pollution, some cities are implementing structures that catch stormwater and clean it before it is released. These structures include retention basins and rain gardens.

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